The present invention is related to an improved method of forming a solid electrolytic capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to a method of improving the electrical and mechanical integrity of a cathode to a cathode lead using a metallurgical adhesive, selected from transient liquid phase sintering and polymer solder adhesives, to form metallurgical bonds
The construction and manufacture of solid electrolytic capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal typically serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the anode, which serve as the dielectric of the capacitor, is typically electrolytically formed to cover at least a majority of the surfaces of the anode. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide and intrinsically conductive polymers such as polyaniline, polypyrrole, polythiophene etc. The solid cathode electrolyte is applied so that it covers at least a majority of the dielectric surfaces. An important feature of the solid cathode electrolyte is that it can be made more resistive by exposure to high temperatures. This feature allows the capacitor to heal leakage sites by Joule heating. The solid electrolyte is typically not readily adhered to a lead frame or circuit trace so in addition to the solid electrolyte the cathode of a solid electrolyte capacitor typically comprises several layers which are external to the solid electrolyte to facilitate adhesion. These layers typically include a carbon layer; a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; a conductive adhesive layer such as solder or a silver adhesive which is then adhered to a highly conductive metal lead frame. It is important that the solid electrolyte be of sufficient buildup and density to prevent the layers overlaying the solid electrolyte from penetrating the solid electrolyte and contacting the dielectric. One reason for this is that these outer layers do not necessarily exhibit the healing properties required for a material directly in contact with the dielectric. Thus, the ability to control the buildup, morphology, uniformity, and density of the solid electrolyte is critical to manufacturing a reliable solid electrolytic capacitor. The various layers of the external cathode also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
In the case of conductive polymer cathodes the conductive polymer is typically applied by chemical oxidation polymerization, electrochemical oxidation polymerization or spray techniques with other less desirable techniques being reported.
The carbon layer serves as a chemical buffer between the solid electrolyte and the silver layer. Critical properties of the carbon layer include adhesion to the underlying layer, wetting of the underlying layer, penetration of the underlying layer, bulk conductivity, interfacial resistance, compatibility with the silver layer, suitable buildup, and suitable mechanical properties.
The silver layer, or a suitable very high conductive layer, serves to conduct current to the lead frame from the areas of the cathode not directly connected to the lead frame. The critical characteristics of this layer are high conductivity, adhesive strength to the carbon layer, wetting of the carbon layer, and suitable mechanical properties. Compatibility with the subsequent layers employed in the assembly and encapsulation of the capacitor are also critical.
An electrically conductive adhesive is used to attach the cathode layer to a lead frame. The electrical properties of the capacitor can be affected if the mechanical integrity of the adhesive/lead frame connection is degraded during assembly and post assembly processing. The adhesive properties of the conductive adhesive, the solder coating on the lead frame, the surface characteristics of lead frame, the coefficient of thermal expansion of the lead frame etc., need to be carefully controlled in order to obtain durable negative connection integrity. The adhesive/lead frame interface is subjected to various thermo mechanical stresses during molding, curing, aging, surface mount testing, solder reflow etc. These thermo mechanical stresses, and the low adhesive strength of the conductive adhesive, often cause a break in the electrical contact between the cathode and lead frame. Adhesives with higher adhesive strengths and lower concentration of conductive particles are able to withstand the stress and maintain mechanical integrity. However, there is trade-off between increasing adhesion and increasing electrical conductivity.
Conductive adhesives are heavily filled with silver particles to get maximum conductivity. Increasing the silver loading will improved the electrical properties but decreases binder/resin concentration in the adhesive which is detrimental to adhesion. Increasing the resin portion will increase adhesion but to the detriment of electrical properties, particularly, conductivity.
U.S. Pat. No. 6,972,943 attempts to circumvent the conflict between adhesion and conductivity of the adhesive by modifying the lead frame surface. The method of the invention in the patent provides grooves and holes in the lead frame so as to have good mechanical integrity between the two surfaces.
U.S. Pat. No. 6,916,433 attempts to improve performance by using conductive fillers with dendrites or protrusions to enhance contact with electrodes and an elastic adhesive resin for enhanced flexibility. The preferred elastic adhesive is a thermosetting resin comprising denatured silicon resin with a dispersed epoxy resin, available from Cemedyne Co. Ltd.
U.S. Pat. No. 7,495,890 discloses a method of improving cathode connection integrity by using secondary adhesives. Although this method improves the cathode connection integrity, higher temperature adhesion performance is limited by the thermal softening temperatures of the polymeric materials in these adhesives.
The polymeric resin in these adhesives helps to form adhesive bonds between the highly conductive cathode layer and the lead frame. One of the weaknesses of the polymeric resin is that they tend to degrade at high temperatures which affects the cathode connection integrity. Another weakness of these metal particle filled adhesives is that the conduction mechanism is percolation assisted by forming a connection between binder coated particles. Due to this binder interference, stable interconnection with the lead frame or between particles is an issue especially when these parts are subjected to thermal, mechanical or environmental stress. On humidity exposure, moisture sorbed by the binders can swell the binders cause an increase in ESR due to increased silver particle to silver particle distance. Silver migration is another issue when conductive adhesive is a silver filled adhesive. Silver migration can lead to an increase in leakage current and increase in ESR. Solders can be used for forming a metallurgical bond between the lead frame and the cathode layer. However, most of the solders available are not suitable for high temperature applications either due to their low melting point or due to the presence of Pb. A need therefore exists for improved reliability cathode connections for high temperature applications.
Through diligent research the present inventors have developed a method of improving high temperature adhesive strength and ESR stability between the lead frame and the cathode layer